This invention relates to the field of well logging and more particular, it relates to a method and apparatus for controlling the effect of contact impedance on a formation resistivity measurement during a logging while drilling operation.
Resistivity logging, which measures the electrical resistivity of earth formations surrounding a borehole, is a commonly used technique for formation evaluation. In general, porous formations having high resistivity are filled with hydrocarbons, while porous formations having low resistivity are water saturated. One technique used to measure formation resistivity is galvanic logging or electrode logging. This resistivity measurement technique uses various arrangements of electrodes on a logging device to generate and measure electrical currents and/or potentials from which one determines formation resistivity.
Various tools and techniques exist that can perform formation evaluations. Some of these tools include logging-while-drilling tools, wireline tools and coiled-tubing tools. As shown by FIG. 1, during formation resistivity measurements, a voltage difference is created between two sections of an electrically conductive tool body, usually a drill collar, separated by an insulating gap 3 (subsequently, insulating gaps will be consistently depicted in gray). By direct conduction, current (shown schematically as 4) flows out of one section of the tool 2, through the borehole 6 and formation 1, and returns to the other section 2. Referring to FIG. 2, radial currents used to compute resistivities are measured directly by ring 7 and button 8 electrodes. The ring electrode comprises a metal band around the tool while the button electrode comprises a metal disc mounted on the collar. the collar. Both ring and button electrodes are electrically isolated from the collar. The collar surrounding an electrode acts as a guard electrode to focus the electrode current into the formation. The surfaces of the electrode and the surrounding collar must be held at the same potential to ensure radially outward current flow. In conventional tool designs, the electronics maintain the potential of the electrode metal at the potential of the collar metal. In the absence of contact impedance effects, this assures that the potentials appearing in the borehole immediately outside the electrodes or the collar are equal. Doing this is necessary to insure that current will flow radially into the formation and not axially along the borehole.
During a resistivity logging operation, an impedance layer can develop at the contact between an electrode and electrolyte (in this case borehole fluid or mud). As a result, impedance layers will appear on the surfaces of the electrode and of the collar. The value of contact impedance depends on a number of factors (electrode material, exposure time, pH, fluid salinity, and frequency) and is highly variable.
The effect of a contact impedance layer is to cause the current to be different from what it would be in a perfect tool without contact impedance. When current flows into an electrode or into the collar through a contact impedance layer, a voltage drop is produced and the potential immediately outside the metal is different from the potential inside the metal. A particularly damaging effect occurs whenever two nearby electrodes (i.e. a button and the collar mass) have different contact impedances. When this situation occurs, the potential immediately outside the button will be different from that immediately outside of the collar mass. This difference causes a current to flow through the shunt resistance supplied by the mud between the electrode and the collar mass. This current is added vectorially to the current passing through the formation. The smaller this shunt resistance, the larger this unwanted current and the more the current differs from what would occur in an ideal tool without contact impedance. Shunt resistance is decreased and the problem of contact impedance increased as gaps between electrodes are reduced and as the mud becomes more conductive.
The demand of mechanical ruggedness in the logging while drilling (LWD) environment limits the size of the gaps. Recent work has shown that differences in contact impedance between the RAB electrodes and collar are often large enough (5-20xcexa9-cm2) that the resulting voltage differences at the tool surface can significantly perturb the current pattern and lead to measurement errors.
In wireline well logging, a logging device suspended from a wireline cable is lowered into the borehole after the drillstring is removed. The logging device makes measurements while the cable is withdrawn. The requirements for mechanical ruggedness are less severe than in the LWD environment and correspondingly, the size and placement of electrodes and insulating gaps are more flexible.
Wireline laterolog tools minimize the effect of contact impedance by employing separate voltage monitoring electrodes to sense the voltage of the mud near the surface of the tool. The monitor electrodes emit essentially no current and so are unaffected by contact impedance. Focusing is achieved by means of a feedback loop that adjusts the bucking or survey current to maintain monitoring electrodes at the same voltage. In theory, this requires infinite amplifier gain but in practice this gain must be limited to guarantee stability. Newer tools used digital measurements together with the principle of superposition to achieve the same ends.
The electrode arrangement of the ALAT wireline tool is described in U.S. Pat. No. 5,396,175 issued to Bronislaw Seeman. Referring to FIGS. 3 and 4, in that device, an array of azimuthal electrodes is incorporated into the conventional Dual Laterolog (DLL) array described in U.S. Pat. No. 3,772,589 issued to Scholberg. As described in the Seeman patent, the intermediate section of the logging device carries a central electrode A0, a first pair of monitor electrodes M1, Mxe2x80x21 connected to each other and disposed on opposite sides of the electrode A0, a second pair of monitor electrodes M2, Mxe2x80x22 disposed on opposite sides of the pair M1, Mxe2x80x21, and a first pair of guard electrodes A1, Axe2x80x21 that are connected to each other and disposed on opposite sides of the pair M2, Mxe2x80x22. The logging device also includes a second pair of guard electrodes A2, Axe2x80x22 that are connected to each other. The A2 electrode has a top portion and a bottom portion. Between the two portions of the electrode A2, an isolated central section carries a pair of annular monitor electrodes M3 and M4 that are electrically connected together. Between the two annular electrodes, there is an array of azimuthal current electrodes Aazi that are spaced apart circumferentially from one another. Each of the azimuthal current electrodes is insulated from the logging device and surrounds an azimuthal monitor electrode Mazi. Each azimuthal monitor electrode is insulated both relative to the logging device and relative to the surrounding azimuthal current electrode.
Formation evaluation electrode resistivity measurements while drilling may be obtained with the Resistivity-at-the-Bit (RAB) tool described in U. S. Pat. Nos. 5,235,285 and 5,339,037. These current RAB logging tools do not address the problem of contact impedance during resistivity logging. There are currently no LWD tools that use monitor electrodes to focus resistivity measurements.
There still remains a need for an apparatus that can account for the effect of contact impedance on a formation resistivity measurement during a LWD operation. This apparatus should also overcome the mechanical limitations of wireline solutions to contact impedance.
It is an object of this invention to provide a resistivity logging tool for use in logging-while-drilling applications that controls the effects of contact impedance on the resistivity measurement.
It is another object of this invention to provide ring and button electrode geometries that can be used to control the effects of contact impedance and can withstand the logging-while-drilling environment.
The present invention provides an apparatus and method to control the effect of contact impedance on a resistivity measurement during a logging-while-drilling operation. The control of contact impedance is accomplished by maintaining a substantially zero difference in potential between two monitor electrodes positioned on the resistivity logging tool in the vicinity of each current electrode. Insulation gaps isolate the monitor electrodes from the current electrodes and the collar. In this design, the current flowing through the current electrode is adjusted such that the voltage difference between the monitor electrodes is zero. One embodiment of the present invention employs monitor electrodes in a ring geometry. In this design, pairs of ring-shaped monitor electrodes are mounted in insulating gaps on the collar on opposite sides of a ring current electrode. Another embodiment of the present invention is a button electrode assembly employing one monitor that is embedded in the center of a button and another monitor embedded in the collar mass and surrounding the button current electrode. These electrode geometries are able to maintain mechanical integrity in a logging-while-drilling environment.